Lithium secondary batteries have a higher energy density in theory compared to other batteries and thus allow to manufacture a small and light-weight battery. Therefore, vigorous studies have been focussed thereon to develop a power source of portable electronic instruments. Particularly, performance of such instruments are even increasing in recent years and their power source is required concominantly therewith to exhibit better discharging characteristics even at a high load. In order to fulfill these requirements, various studies are in progress next to the prior art battery using nonaqueous electrolyte solutions referred to as lithium ion battery to develop a battery using a polymer electrolyte that functions both as the nonaqueous electrolyte solution and the polymer separator of the prior art battery. Much interest has been focussed to a lithium secondary battery using the polymer electrolyte because of its remarkable advantages such as the possibility of making the battery smaller and thinner in size and lighter in weight as well as leak free.
A known type of the polymer electrolyte is comprised of a matrix of ion-conductive polymer retaining the nonaqueous electrolyte solution therein produced by polymerizing and cross-linking a precursor monomer of the ion-conductive polymer in the nonaqueous electrolyte solution. Whether the polymerization is heat polymerization or photopolymerization, a polymerization intiator must be used to generate a polymerization initiating specis. It was discovered, however, that some initiators may cause a chemical reaction to adversely affect the battery performance including the discharging capacity under load and charge-discharge cycle characteristics during repeated cycles when the initiator remains after the polymerization.
Since a certain quantity of the initiator is indispensable to produce a polymer electrolyte gel having a satisfactory mechanical strength, it is practically inevitable for the polymer electrolyte to contain an amount of residual initiator. JP-A-1015848 discloses to decrease the level of residual initiator by decomposing thermally or ultrasonically. However, it is practically impossible to completely decompose the initiator by this treatment and the problem of residual initiator still remains to exist and the battery performance will be adversely affected depending on the nature of initiator used as discussed above.
JP-A-8287890 disclsoes that when a phosphine oxide-based initiator is used in a photocurable resin applied to the sealing areas of battery openings and the insulating areas of both anode and cathode terminals, it is possible to improve the strength of the sealing area and also the insulation of anode and cathode. This proposal is not related to the improvement of the performance of polymer electrolyte.
Accordingly, the problem to be solved by the present invention is to eliminate or ameliorate any adverse effect of the residual initiator on the battery performance.